Known electrophotographic processes, which are used in all modern electrophotographic copying devices, are typically cyclical processes, each cycle carrying out all sequential stages of the entire copy production process. In other words, to produce each copy from an original document, the entire cycle of the electrophotographic process is repeated. Generally, such a cycle includes the following stages:
(1) charging the photosensitive surface of an intermediate information carrier (photoreceptor); PA1 (2) exposure of the charged surface, i.e., recording an image of the original document onto the charged surface of the photoreceptor, thereby forming a latent electrostatic image on this surface; PA1 (3) developing the exposed surface of the photoreceptor; PA1 (4) transfer and fixation of the obtained image on a final carrier (a copy), e.g. a paper; and also the following step PA1 (5) neutralizing the residual charges and mechanically cleaning the photoreceptor surface to prepare it for the next cycle. PA1 a preliminary stage (master-making steps) stipulating thermal treatment of the layer to give photosemiconductive properties thereto, which is performed prior to the copying process; and PA1 a recovery stage performed after the completeness of the copying process for returning the layer into its initial state. PA1 (a) forming a latent electrostatic image of the copying information on an intermediate information carrier in the form of a photoreceptor by means of said recording photoreceptor, wherein the intermediate information carrier is such that it, whilst being charged, stores the charge during a period of time required for producing a desired number of copies of the original information; PA1 (b) developing said latent electrostatic image on the intermediate information carrier; PA1 (c) transforming the developed image from the intermediate information carrier onto a final information carrier; and PA1 (d) fixing the transformed image on the final information carrier. PA1 uniformly charging the surface of the intermediate information carrier with charges of the opposite polarity with respect to the charges of the image elements formed on the recording photoreceptor, wherein said charging is carried out with simultaneous illumination of the surface of the intermediate information carrier; PA1 engaging the surfaces of the recording photoreceptor and the intermediate information carrier, and subsequently disengaging these surfaces; and PA1 illuminating the photosemiconductor layer of the disengaged intermediate information carrier. PA1 forming a latent electrostatic image on the surface of an intermediate information carrier, said latent electrostatic image being in the form of a potential relief of said surface, wherein said intermediate information carrier comprises a photosemiconductor layer with a dielectric coating, and is such that it, whilst being charged, stores the charge during a period of time required for producing a desired number of copies; PA1 maintaining said potential relief at a given level during the entire process of copying; PA1 developing said latent electrostatic image on the intermediate information carrier; PA1 transforming the developed image from the intermediate information carrier onto a final information carrier; and PA1 fixing the transformed image on the final information carrier. PA1 (i) a cylindrically shaped recording photoreceptor mounted so as to be radially displaceable between its operative and operative positions; PA1 (ii) a recording system for electrostatic recording of an electrostatic image of said original information onto the recording photoreceptor; PA1 (iii) a cylindrically shaped rotatable intermediate information carrier in the form of a photoreceptor, which is such that it, whilst being charged, stores the charge during a period of time required for producing said desired number of copies; PA1 (iv) an imaging system for forming a latent electrostatic image of the copying information on the intermediate information carrier by means of the recording photoreceptor; PA1 (v) a developer tool for developing the latent electrostatic image on the intermediate information carrier; PA1 (vi) a transfer means for transferring the developed image onto a final information carrier; and PA1 (vii) a fixating means for fixating the transferred image on the final information carrier.
A copying apparatus based on the known electrophotographic processes has limited throughput. This is due to the inertness of an image scanning system applied for exposing the original document, and the so-called "fatigue" of a photosemiconductor layer under the cyclic stresses thereof in a high-speed charge/discharge mode. Additionally, mechanical wear of the photoreceptor occurs caused by the permanent operation of the cleaner.
In a classical electrophotographic process, storage of the latent electrostatic image for multiple-copy production utilizing single exposure is very difficult, due to the existence of so-called "dark potential decay" on the surface of the photoreceptor. The "dark potential decay" is a phenomenon consisting of time discharge of the photosemiconductor layer in darkness caused by the inherent dark conductivity of the photosemiconductor layer. This dark potential decay is characterized by the exponential nature of the discharge process dynamics, namely it is especially pronounced during the first few seconds after the charging of the photosemiconductor layer. For example, in the case of amorphous Se photosemiconductor layer, the surface potential decreases in darkness from the initial value of 700V to the value of 600V during the first 10 seconds. The more sensitivity of the photosemiconductor layers (e.g., SeTe or As.sub.2 Se.sub.3), the more the dark potential drop, thus making the application of such photosemiconductors impossible in a classical electrophotographic process for production of a plurality of copies from a single exposure.
In addition to the fact that the dark discharge inside a photosemiconductor layer (i.e., passive discharge) reduces the potential level of a latent electrostatic image, the surface potential value is also affected by external factors, such as the conditions of the development and image transfer processes. The influence of external factors is essentially pronounced in a classical electrophotographic process, wherein surface charges are held solely by the compensating charges of the opposite polarity induced in a conductive substance, which charges have the tendency to flow. This is the reason that the external effect causes an even more intensive (active) discharge of the surface of the photoreceptor, and, consequently, reduction of the potential contrast, which negatively affects the quality of obtained copies.
Attempts have been made to produce a plurality of copies using the classical electrophotographic process. U.S. Pat. No. 4,286,865 discloses a copying apparatus in which a special system for controlling the copying process is used to perform the process in batches of ten copies from one image exposure. When more than 10 copies of the same original are required, the exposure is repeated after each ten copies.
The ability of this process is limited, because both the exposure speed and the process speed (i.e., speed of image reproduction) equal to the exposure speed, and, consequently, the throughput of the apparatus, directly depend on the sensitivity of a photosemiconductor layer, which cannot be sufficiently high in this case. Indeed, the increase in the photosemiconductor layer sensitivity causes acceleration of the dark potential decay, and makes it practically incapable of providing stable conditions for the potential relief of the latent electrostatic image at the level of its initial recording.
Techniques of electrophotographic copying are disclosed, for example, in U.S. Pat. Nos. 4,297,422, 4,297,423 and 4,442,191. According to these techniques, the charge storage on the surface of a photoreceptor is typically provided after the image transfer.
More specifically, in the electrophotographic process disclosed in U.S. Pat. No. '422, the image development stage is followed by initial frame illumination on the photoreceptor, and then, with the concurrent electrization during the image transfer, secondary illumination is performed through a material onto which the developed image is transferred. As specifically indicated in this patent, the charges are to be preserved within shadow sections of the photoreceptor, i.e., within the regions where the developer was present before the image transfer. It is assumed here that image sections are transparent for charge passage therethrough, but are opaque for light passage. This approach does not take into account the dark potential decay within the image sections. It is evident that such a technique is not capable of providing a stable and equal charge value for the production of a next copy.
U.S. Pat. No. '423 describes two examples of the charge storage technique on the surface of a multi-layer organic photoreceptor. According to one example, the charges of a latent electrostatic image on the photoreceptor are neutralized by the charges of the opposite polarity, by means of a preliminary chargeable dielectric layer-neutralizer, and contact recharge of the photoreceptor within the blanked regions of the image is simultaneously carried out. In other words, a negative latent electrostatic image is created, and, thereafter, reverse development and image transfer are performed. According to the other example, the development stage is followed by uniform recharge of the developed surface of the photoreceptor within the entire frame with the corona of the opposite polarity. Thereafter, the frame illumination with the charge neutralization within the blanked regions, and the cleaning of the photoreceptor surface from the developer are performed. Both of the described methods cannot be used for a high-quality high-speed copying process. Moreover, a low lifetime of the organic photoreceptor does not allow its operation in a high-speed copying mode.
Another technique, aimed at storing charges of a latent electrostatic image, utilizes a "locking" dielectric layer accommodated inside a multi-layer photoreceptor structure. For example, according to the disclosure in U.S. Pat. No. 4,407,918, such a dielectric layer is interposed between two photosemiconductor layers of different spectral sensitivity. The main drawback of this approach is that it requires a complicated multiple-operation process associated with the use of a special means of ultraviolet illumination. Moreover, a very thin (2 .mu.m) surface photosemiconductor layer makes the photoreceptor not sufficiently strong, and consequently, unsuitable for operation under the conditions of the process of multiple-copies production.
U.S. Pat. No. 4,898,797 discloses the use of a multi-layer photoreceptor, in which a dielectric layer is interposed between a conductive substrate and a photosemiconductor layer. In this case, a latent electrostatic image is formed not on the photoreceptor surface, but rather at the inside of the photoreceptor structure, on the boundary dielectric-photosemiconductor. This does not provide a high-quality image reproduction, owing to the fact that the development proceeds through an insulating layer of the photosemiconductor. Moreover, the surface of the photoreceptor layer does not posses sufficient mechanical strength for the multiple-copies production process.
Another technique of the kind specified is disclosed in US Pat. No. 5,053,304. Here, however, a technologically complicated multi-layer photoreceptor is used, which also suffers from the above-described disadvantages.
A detailed analysis of existing techniques aimed at enabling the multiple-copies production is disclosed in the article "A Review of Electrographic Printing", D. E. Bugner, Journal of Imaging Science, Volume 35, Number 6, November/December 1991.
The existence of various attempts, directed towards finding the ways of applying the electrophotographic means for operating in the multiple-copies production mode, are associated with the following. The electrophotographic method of multiple-copy production, based on the use of an intermediate information carrier as an electrostatic matrix, is characterized by such advantages (as compared to the other known methods), as operative multiple-copy production and multiple usage of the matrix. The indispensable conditions of the multiple-copy production process include the storage of a charged image on the electrostatic matrix during the entire process of copying, and also physical and mechanical wear-stableness of the matrix.
One of the approaches of applying a wear-stable intermediate information carrier consists of the use of the known TESI-process in its various embodiments, described, for example, in "Electrophotography", R. M. Schaffert, Focal Press., London, 1975. The main concept of the TESI-process consists of the transfer of surface charges of a latent electrostatic image from a photosemiconductor surface to a dielectric one, which is then used as an intermediate information carrier. The most essential drawbacks of this approach include the complexity of its practical application, owing to the fact that conditions of sequential realization of separate stages of the electrostatic image transfer are not the same, which makes it impossible to provide continuity of the copying process in an automatic mode. Additionally, due to the redistribution of charges between the photosemiconductor and dielectric layers (in accordance with their thicknesses, dielectric constant and other characteristics, as well as the conditions of their interaction), the complete and high-quality transfer of the latent electrostatic image cannot be provided. This negatively affects the quality of the so obtained copies, especially during the reproduction of half-tones, because of distortion of the gradation of images of different densities.
The aspiration to create an electrostatic matrix stable for multiple-copy production led to the development of the known NHEP Xeroprinting Technology, disclosed, for example, in the article "A New Digital Color Xeroprinting Technology", Journal of Imaging Science and Technology, vol. 36, No 1, January/February 1992. This technology is based on the use of an electrostatic matrix with a specially developed photo-thermo-sensitive layer capable of providing a relatively high stableness for multiple-copies production, as compared to the earlier techniques. However, whilst realizing one of the advantages of the electrophotographic copying method, namely, the multiple use of the electrostatic matrix, NHEP Xeroprinting Technology fails to provide such an advantageous feature of the electrophotography as the operativeness of multiple-copy production, thereby approaching the usual mode of the offset printing. This is due to the fact that the latent electrostatic image formation on the matrix (an intermediate information carrier) requires the following operational stages:
Thermal treatment, used for imparting the sensitivity of the layer and recovering the layer, in addition to special means and conditions, needs time for heating and cooling the layer, and is therefore an inertial process. Matrices for multiple use are actually manufactured separately, under special conditions, which does not enable a continuous copying process to be provided. The situation even worsens when manufacturing four-color matrices, which are usually required for the copying in colors.
In view of the above, it is evident that NHEP Xeroprinting Technology, although being advantageous in some aspects as compared to all other conventional techniques, is not applicable for operative copying. For the above reasons, the known developed techniques aimed at providing the ability for multiple-copies production are not applicable in any one of the existing copying-multiplying systems.